The present invention relates to a polarization compensator used to compensate for the depolarization effects due to geometric, thermal, and material birefringence in an optical system. An optical system that utilizes polarization as the means for contrast control will benefit from this method of compensation.
It is well known that the polarization of light within an optical system is greatly influenced by the incident angle the light makes with respect to the different surfaces in the optical system. This results from the different Fresnel reflection coefficients for each polarization state. Polarization defects may be manifested as the xe2x80x9cMaltese Crossxe2x80x9d or thermal birefringence.
For example, in one optical system such as a liquid crystal light valve projector, an illumination source shines light onto a polarizer that reflects s polarized light to a liquid crystal display (LCD) panel, a also referred to as a liquid crystal light valve (LCLV). If the LCD is fully activated, then the LCD converts the s polarized light to p polarized light after it is reflected by a dielectric mirror behind the LCD. The p polarized light is reflected back to the polarizer, where it passes through the polarizer to the projection screen. If the LCD segment is not activated, then it does not change the polarization of the light and the light remains s polarized. The s polarized light is reflected back to the polarizing surface which in turn reflects the light back towards the illumination source. In a perfect system, 100% of the light reflected from the polarizer to the LCD is s polarized light. However, the polarizer does not reflect 100% s polarized light because of geometrically induced errors in the polarizer. Due to the cone of light that is incident on the LCD, all of the light in the cone is not polarized in the s direction by the polarizer with respect to the LCD. There are small rotations in polarization direction due to the angle of incidence of the light directed onto the polarizing surface. In this situation a higher degree of twist in the polarization axis occurs for a ray with a higher angle of incidence. The direction of rotation of the polarization away from the ideal is dependent upon the angle of incidence of the ray. For example, a ray incident at 5xc2x0 might rotate the polarization axis by 2xc2x0 while a ray incident at xe2x88x925xc2x0 would rotate the polarization axis by xe2x88x922xc2x0.
The light rays with polarizations that have been slightly rotated away from the ideal are reflected by the dielectric mirror in the LCD and return to the polarizer with their polarization axis still slightly if rotated. This causes a slight percentage of these rays to plunge into the polarizer instead of being reflected by the polarizer. This small percentage of light is then projected onto the projection screen. Thus, when the LCD is completely off or inactivated, a small percentage of the light reflected off the LCD will not be completely polarized in the s direction with respect to the polarizing surface. This causes unwanted light to pass through the polarizer and slightly illuminate the screen, causing the xe2x80x9cdarkxe2x80x9d state to be slightly illuminated which reduces the contrast ratio of the projection system.
Another problem associated with a dark or off state of the LCD projector is the problem of birefringence introduced by thermal gradients across the counter electrode substrate. The LCD itself, including the substrate, is packaged in an anodized aluminum case to reduce reflections that cause unwanted light scattering. Thus, light striking the case holding the LCD is absorbed, creating heat in the LCD package. This heat is transferred to the counter electrode substrate which causes the temperature to rise in the substrate, creating a thermal gradient across the substrate. This thermal gradient creates birefringence, causing the dark state or off state of the LCD to be xe2x80x9clighterxe2x80x9d than desired. Therefore, when the LCD is completely turned off, what should be a totally black screen will actually have some light projected on the screen, causing the screen to appear gray instead of a deep black. Therefore, it would be desirable to eliminate the birefringence caused by the thermal gradients across the counter electrode substrate and thereby improve the contrast ratio by making the dark or off state of the LCD appear black instead of gray on the projection screen.
The primary conventional approach to polarization compensation is to use a birefringent film, typically quarter wave retardation films for reflective LCD panels and half wave retardation films for transmissive LCD panels. Examples of such systems include U.S. Pat. No. 5,459,593 and U.S. Pat. No. 5,576,854. However, this is only a partial solution to the problem, because this approach does not fully compensate for all of the different off-axis rays, and slightly different polarization states and orientation that is caused by the optical components in the system. In addition, because this system is passive, i.e. the retardance value of the film is fixed, the retardance cannot be changed to correct for current operating conditions, or to optimize performance of the system.
What is therefore desired is a projection display having a polarization compensator that is capable of providing improved polarization correction, that improves the contrast of the projection display system without sacrificing brightness, that corrects local polarization defects, and that allows an active correction of polarization defects to optimize correction of polarization defects based on current operating conditions.
The present invention overcomes the drawbacks of the prior art by providing a projection display system having a polarization compensator. The projection display system includes a light source, at least one polarizer, at least one liquid crystal panel, and a projection lens for projecting an image. The polarization compensator has a plurality of regions each having a respective birefringence. The birefringence of one of the regions is different than the birefringence of another of the regions.
The present invention also provides a method for displaying an image. The method comprises the steps of providing polarized light, generating an image from the polarized light, correcting a plurality of polarization defects of the light by correcting at one location one polarization defect and correcting at another location another polarization defect, and projecting the image.
The various aspects of the present invention have one or more of the following advantages. The polarization compensator is capable of correcting polarization defects on a local basis, in contrast to the prior art systems which allow only a single, uniform correction provided by a single, passive retardation film. By correcting polarization defects locally, the system achieves greater contrast without sacrificing brightness. The present invention also provides embodiments which allow the polarization defects to be corrected locally through the use of electronically controllable birefringence. This provides for greater control over the correction of polarization defects. It also allows the birefringence to be changed locally over time, so that the polarization defects can be changed as needed at different times. This also enables the use of a feedback mechanism to automatically adjust the polarization compensator to yield the best system contrast under all operational condition.
The foregoing and other objectives, features, and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.